Key points are not available for this paper at this time.
Lieven Verschaffel, Erik De Corte, Heidi Vierstraete, Upper Elementary School Pupils' Difficulties in Modeling and Solving Nonstandard Additive Word Problems Involving Ordinal Numbers, Journal for Research in Mathematics Education, Vol. 30, No. 3 (May, 1999), pp. 265-285
Verschaffel et al. (Sat,) studied this question.